Elevated mRNA levels of major histocompatibility complex class II genes in lymphocytes of autoimmune BB rats.

The BB rat spontaneously develops autoimmune abnormalities such as insulin-dependent diabetes mellitus and thyroiditis. The autoimmunity of the BB rat is controlled in part by genes of the major histocompatibility complex (MHC), known as the RT1 complex in the rat, and accumulating evidence suggests the involvement of MHC class II molecules. The RT1 complex specifies two types of class II molecules, which are encoded by the loci RT1.B and RT1.D. We have determined the relative steady-state mRNA levels of the class II genes RT1.B beta, RT1.D alpha, and RT1.D beta in splenic lymphocytes from individual autoimmune BB rats of various ages and from age-matched histocompatible normal Wistar-Furth (WF) rats. The relative steady-state mRNA levels of the RT1.D alpha and RT1.D beta genes, but not of the RT1.B beta gene, were elevated approximately 2.5-fold in lymphocytes of prediabetic BB rats 45-75 days old in comparison with age-matched normal WF rats and older BB rats greater than 75 days old. In the diabetic and nondiabetic BB rats greater than 75 days old, the RT1.D alpha and RT1.D beta transcripts were found at lower normal levels, similar to that of WF rats. In contrast, the RT1.B beta transcripts were found at comparable levels in lymphocytes of the BB and WF rats at all ages examined. The increased steady-state mRNA levels of the RT1.D alpha and RT1.D beta genes in the prediabetic BB rats may reflect differences in the proportion of lymphocytes expressing these genes and thus differences in splenic lymphocyte populations.(ABSTRACT TRUNCATED AT 250 WORDS)